Presently, research and development of a nonaqueous electrolyte secondary battery in which charging and discharging are performed by moving Li ions between a negative electrode and a positive electrode, as a high energy density battery, have been actively conducted. Until now, a lithium ion secondary battery which includes a positive electrode including LiCoO2 or LiMn2O4 as an active material and a negative electrode including a carbonaceous material that inserts and extracts lithium has been widely commercialized for mobile devices.
The lithium ion secondary battery has been recently and widely used as power sources for environmental-friendly automobiles such as electric vehicles (EV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), and vehicles equipped with an idling-stop system (ISS), which are developed in view of environmental issues. When the lithium ion secondary battery is mounted on a vehicle such as an electric vehicle or a hybrid electric vehicle, the lithium ion secondary battery is required to have storage performance under high temperature environment, cycle performance, high power with long-term reliability and the like.
In addition, when the lithium ion secondary battery is installed in an engine compartment of an automobile to be used as a substitute for a lead storage battery, it is required for the lithium ion secondary battery to have high temperature durability (for example, at 60° C. or more, and preferably, 80° C. or more). Further, when high performance in cold regions is required, high power performance or long life performance at low temperature environment (for example, −30° C.) is needed.
Accordingly, constituent materials of batteries such as a positive electrode, a negative electrode, a separator, and a nonaqueous electrolyte are required to be composed of materials having excellent chemical and electrochemical stability, strength, and corrosion resistance at high temperature and low temperature.
However, a negative electrode used in a nonaqueous electrolyte battery such as a lithium ion secondary battery has generally a structure in which a negative electrode active material layer is formed on a current collector. In addition to a negative electrode active material, a binder for binding the negative electrode active materials to each other and for binding the negative electrode active material and the current collector is used in the negative electrode active material layer.
As the binder, a fluorine-based resin (for example, polyvinylidene fluoride (PVdF)) or a modified substance thereof is generally used. However, the fluorine-based resin or the modified substance thereof easily swells with respect to the nonaqueous electrolyte at a high temperature. Accordingly, there is concern that high temperature cycle performance may be deteriorated in a battery having a negative electrode including the fluorine-based resin or the modified substance thereof as the binder. Specifically, in the battery using such a fluorine-based resin as the binder for the negative electrode, network of an electron conduction of the negative electrode is disconnected as charge-and-discharge cycles proceed at a high temperature, and as a result, internal resistance of the negative electrode is increased.
Therefore, a synthetic rubbery polymer such as an acrylic resin, instead of the fluorine-based resin such as PVdF, is proposed as the binder for the negative electrode.